In recent years, polymeric materials have gained widespread theoretical interest and practical use in many fields [G. Harsanyi. Materials Chemistry and Physics Vol. 43, Issue 3, 1996, 199]. Conducting polymers in particular have found increasing use in the field of biosensing, where conducting polymers provide a unique function as an interface between smart sensors and intelligent molecular receptors. Of all the known conducting polymers, such as ionically conducting polymers, charge transfer polymers and conjugated conducting polymers, redox polymers are by far most widely used in biosensing applications.
Glucose sensing, an area in biosensing which has been undergoing significant research in recent years, relies on electron mediation of enzymatic oxidation of glucose to gluconic acid by glucose oxidase is required. The electron mediating function of redox polymers has been widely studied and applied to many amperometric glucose biosensors.
In its natural enzymatic reaction, co-enzyme flavin adenine dinucleotide (FAD) is an electron carrier present in glucose oxidase is reduced to FADH2 (reduced form of FAD) and oxidized back to FAD by molecular oxygen. O2 is then reduced to H2O2. This cyclic oxidation and reduction enables FAD to act as an electron acceptor. Since neither glucose nor gluconic acid is electro-active within the working potential window from −0.5 to 1.0 V, either the increase in H2O2 concentration or the decrease in O2 concentration is being measured to quantify the glucose concentration.
However, the accuracy of measurements based on the measurement of H2O2 and O2 is compromised because firstly, the partial pressure of atmospheric O2 affects amperometric response, and secondly, the quantitative measurement of O2 at high glucose concentration is difficult because O2 is used up as the sensing proceeds. The detection of H2O2 by its oxidation at a platinum electrode requires a working potential of 0.5 to 0.6 V (vs. Ag/AgCl), and thus is subjected to interferences of electro-active species in blood, such as ascorbic acid and uric acid which are electrochemically active at this potential.
To circumvent the above-mentioned problems associated with glucose monitoring involving O2 or H2O2, redox-active mediators have been proposed as artificial electron acceptors in place of oxygen molecules for FADH2.
A successful mediator should, in principle, meet three requirements: (1) fast electron-exchange rate with enzyme and electrode, (2) stable attachment to the electrode and (3) processable in aqueous medium.
For this reason, two groups of mediators were extensively investigated, namely, transition metal complexes and ferrocenyl materials. In recent years, many groups have focused their attention on the synthesis and biosensing applications of ferrocenyl materials, both monomeric and polymeric. For example, polyferrocenyl compounds have been used as redox indicators in molecular recognition [J. E. Kinston, et al, J. Chem. Soc., Dalton. Trans (1999) 251.], as mediators in biosensors [S. Koide, et al, J. Electroanal. Chem., 468(1999) 193.] and as coating to modified electrode surface [S. Niate, et al, Chem. Commun., (2000) 417.]. However, most of the known ferrocenyl materials are only soluble in non-polar media, only few ferrocenyl and polyferrocenyl materials are water-soluble [O. Hatozaki, et al, J. Phys. Chem., 199 (1996) 8448.]. Water-soluble ferrocenyl materials are of particular interest as redox mediators in biosensing. By co-polymerizing alkene substituted ferrocenes, such as vinylferrocene, with an appropriate water-soluble polymer, it is possible to prepare ferrocenyl materials that are readily soluble in water. But it has been shown that the free radical initiated polymerization of vinylferrocene is unusual [A. J. Tinker, et al, J. Polym Sci., Polym. Chem. Ed., 13 (1975) 2133; M. H. George, et al, J. Polym Sci., Polym. Chem. Ed., 14 (1975) 475.]. Co-polymerization of vinylferrocene is known to be difficult because the ferrocenium is a radical scavenger in the polymerization system, resulting in that the reaction does not obey normal radical polymerization kinetics. Termination of the polymerization reaction occurs by an intramolecular electron transfer from a ferrocene nucleus to the growing chain radical. This leads to the deactivation of the polymer chain and a polymer which contains a high spin Fe(III) species.
Polyacrylamide has been widely used as support matrix in enzyme immobilization and biosensing because of its good chemical and mechanical stability and its inertness to microbial degradation [I, Willner, et al, J. Am. Chem. Soc., 112 (1990) 6438.]. However, attempts of co-polymerization of vinylferrocene and acrylamide and its derivatives were not successful [H. Bu, et al, Anal. Chem., 67 (1995) 4071 and references therein.]. Instead, to by-pass the inefficient co-polymerization of vinylferrocene, chemical grafting procedures were proposed in preparing ferrocenyl materials [S. Koide, et al, J. Electroanal. Chem., 468 (1999) 193; J. Hodak, et al, Langmuir, 13 (1997) 2708; A. Salmon, et al, J. Organomet. Chem., 637-639 (2001) 595.]. In two recent reports [N. Kuramoto, et al, Polymer 39 (1998) 669; H. Ahmad, et al, Colloids and Surfaces, 186 (2001) 221], vinylferrocene co-polymers were synthesized, but minute loading of ferrocene and lack of cross-linkable groups in these polymers restrict their use in biosensors.
Commercially available biosensors include those manufactured by Therasense Inc. (cf., for example U.S. Pat. No. 6,338,790), Inverness Medical Technology (cf., for example U.S. Pat. No. 6,241,862) and Matsushita Electric (cf., for example, U.S. Pat. No. 6,547,954).
Therefore, there remains the need for vinylferrocene-based polymeric mediators having superior performance characteristics. Consequently, it is a goal of the present invention to develop new methods of synthesis for new vinylferrocene-based polymeric mediators. It is also a goal of this invention to provide biosensors with enhanced performance, and which would impose minimal inconvenience to the end user of the biosensor as much as possible.
These goals are solved by the various aspects of the present invention, namely the sensors, membranes, polymers, and processes as defined in the respective independent claims.